“SOI” is the same as “silicon-on-insulator”. “MOSFET” is the same as “metal-oxide-semiconductor-field-effect-transistor”. The conventional MOSFET has a structure in which a MOSFET is fabricated on a silicon-substrate, as shown in FIG. 1. The MOSFET comprises a source, gate and drain on the silicon-substrate (1). The gate (1a) consists of a channel, SiO2-layer on the channel and gate-electrode (3) on the SiO2-layer. An assembly of the source, channel and drain is called a silicon-membrane (2). An assembly of the source, gate and drain is called a transistor (1b). “SOI-MOSFET” means a MOSFET fabricated with the SOI. The conventional SOI-MOSFET has a structure in which a MOSFET is fabricated with the SOI, as shown in FIG. 2. “Depletion” means a state in which carriers such as electrons or positive holes are absent in the gate. “Partial depletion” means a state in which depletion is partial. “Complete depletion” means a state in which depletion is complete.
Positive charge is generated in the BOX when exposed with X-ray radiation. The SOI-MOSFET is less radiation-resistant against appreciable amounts of X-ray irradiation because radiation-induced positive charge in the BOX has ill effects on the transistor characteristics.
In order to solve the above problems, several methods which apply negative voltage to a bottom of the silicon-substrate, a back-gate, have been presented (Patent Literatures 1-3).
The Patent Literature 1 discloses a method for setting voltage applied to the back-gate as a function of radiation-exposition times. As shown in FIG. 2, the conventional SOI-MOSFET has a thick BOX (4) buried in a very thick silicon-substrate (1). For example, the silicon-substrate (1) is μm thick and the BOX (4) is sub-μm thick. This causes secondary problems: a very large voltage for long periods of time is required for canceling the positive charge in the BOX; discontinuous or non-constant radiation makes the performance of the SOI-transistor instable, due to overhigh or overlow voltage. The conventional SOI-MOSFET may be destroyed due to such a large voltage for long periods of time. However, the method disclosed by the Patent Literature 1 cannot resolve the above problems. The positive charge in the BOX cannot be removed with ease by applying voltage to the back-gate.
The Patent Literature 2 discloses a SOI-MOSFET that is capable of inhibiting leak-current independently with the gate-control. However, the SOI-MOSFET is essentially less resistant against radiation-exposure due to no method to cancel radiation-induced positive charge.
The Patent Literature 3 discloses the SOI-MOSFET that is capable of increasing or decreasing a threshold voltage by using high bias-substrate-coefficient on stand-by or line.
The present invention differs from the Patent Literatures 1-3 in respect to the structure and function with each other, as follows.
1) The SOI-MOSFET disclosed in the Patent Literature 1 has no system to detect the radiation-induced positive charge. Contrary, the present invention has a structure that detects a voltage threshold shift due to the radiation-induced positive charge followed by application of voltage to cancel the radiation-induced positive charge.
2) The SOI-MOSFET disclosed in the Patent Literature 2 has a well-in-well including a p-well and a n-well. Contrary, the present invention has any one of p-well or n-well but does not have the both.
3) The SOI-MOSFET disclosed in the Patent Literature 2 installs a circuit connecting the well-in-well and the back-gate in order to inhibit leak-current in the gate of the MOSFET. Contrary, the present invention mounts a circuit connecting a gate, a voltage-source and a via that penetrates the BOX to lead a buried p-well, wherein a voltage of the voltage-source is applied to the buried p-well, negative electron of the buried p-well generates, electron-tunneling to the BOX takes place, and positive charge in the BOX is canceled with the negative electron.
4) The SOI-MOSFET disclosed in the Patent-Literature 3 is able to increase a threshold voltage but is not able to detect a voltage threshold shift. Contrary, the present invention has a structure that detects a voltage threshold shift due to the radiation-induced positive charge followed by application of voltage to cancel the radiation-induced positive charge.